Another solution to reduce the pilot overhead is to directly estimate the corresponding cascaded channels by utilizing the multi-user correlation. Since all users communicate with the BS via the same RIS, the cascaded channels \(\lbrace {{\bf {H}}_k}\rbrace _{k=1}^{K}\)  associated with different users have some correlations. Thus, this multi-user correlation can be exploited to reduce the pilot overhead required by the cascaded channel estimation [1]}.
